Commonly assigned U.S. Pat. Nos. 5,682,233; 5,582,184; and 5,823,973 teach methods and apparatuses for testing for body fluid constituents. For example, these patents teach method and apparatus for determining a level of blood glucose in a minimally invasive manner by testing for glucose in interstitial fluid. More specifically, these patents teach a method for drawing a sample of substantially blood-free interstitial fluid and subsequently testing the sample for constituents. The testing may be done in any one of a number of ways (e.g., colormetric or electro-chemical testing). A preferred testing method is identified as infrared (IR) absorption testing.
In IR absorption testing, an IR source directs a band of IR wavelengths to a collected sample. Certain wavelengths (e.g., 1040 cm−1) are absorbed by glucose. The amount of such absorption provides an indication of the amount of glucose in the sample. In turn, this information permits calculating the patient's blood glucose level.
In addition to containing IR absorbing glucose (or other desired constituent to be tested), the sample may contain other elements that absorb IR in the same spectral range that glucose absorbs IR energy. For example, protein and blood cells absorb IR in such a spectral range. In fact, these components absorb a greater portion of the IR radiation than the glucose to be measured. IR absorption by these components complicates attempts to measure the glucose in a sample. Providing techniques to draw a substantially blood free sample (or otherwise filtering blood cells out of the sample) relieves the complexity. However, remaining components (e.g., protein) continue to have a substantial influence on the amount of IR being absorbed by the sample. Further, water in the sample absorbs a very high portion of the IR energy.
Therefore, there is a need for an apparatus and method that detects and measures a body fluid analyte by spectral testing in a sample containing other constituents that absorb light wavelengths (visible or invisible) in the same spectral range in which the desired body fluid analyte absorbs light wavelengths.